This invention relates generally to methods for etching treatment of semiconductor structures and more particularly to microwave plasma anisotropic dry etching of thin film semiconductor structures and, specifically, to microwave generated plasma methods of etching II-VI compound semiconductor thin films or substrates employing a halogen reactive gas medium to chemically etch patterned semiconductor layers or substrates of semiconductor structures without significant surface or structural damage.
The manufacture of semiconductor devices and structures usually requires the selective etching of particular patterns in specific layers thereof. Previously, most etching of this type was wet etching, i.e., employing wet chemical materials that are applied to the patterned surface. In wet etching of II-VI compound semiconductors, the etching solutions primarily used are a solution of sodium hydroxide; hydrochloric acid; and a compound solution of nitric acid, hydrochloric acid, and water. These etching solutions are used at appropriate temperatures and with appropriate composition ratios as known in the art to achieve desired etching rate. However, the use of wet etching is isotropic in nature so that the resultant linewidth and pattern resolution of the wet etched pattern is not the same as the originally desired resist pattern. Further wet etching requires additional treatment steps of rinse and drying. Because of these mentioned factors, semiconductor structure yields are not uniform or high.
A common problem relative to wet etching is the lack of reproducibility. A predetermined etching rate cannot be achieved unless the temperature and the composition of etching solution are closely controlled. Also, where the etching solution contains a volatile material, the composition of the solution significantly changes over time. Therefore, etching rate of the wet etchant at the time when the solution is prepared and etching rate of the wet etchant at some later time are dramatically different. Furthermore, in wet etching, the pattern to be etched in a semiconductor layer or film cannot be formed to be the same as pattern of the mask because the etching is isotropic in nature, including side etching, resulting in a larger overall etched pattern. Also, the etching of patterns of processed sections are limited, for example, formation of vertical sections or deep vertical grooves with large length/breadth ratio is difficult to obtain.
Wet etching of Group II-V group compound semiconductors present more problems than wet etching of other semiconductors, such as, Group III-V group compound semiconductors. For example, in the case wherein etching of ZnSe is performed employing a hydrochloric acid and nitric acid etching solution, the etching solution penetrates into ZnSe and it is very difficult to completely remove etchant even with long periods of rinsing. As a result, there is a substantial degradation of film characteristics. Also, in the case where etching of ZnSe or ZnS.sub.x Se.sub.1-x (0&lt;x.ltoreq.1) is performed in the solution of NaOH, the surface morphology worsens extremely, therefore, these compounds are not particularly suitable for precision wet etching compared to Group III-V compound semiconductors. In the case where hydrochloric acid is employed as an etchant, the etching rate is very slow and, therefore, HCl is not practical for use in the fabrication and selective etching of Group II-VI compound semiconductors.
For many years, sputtering and ion milling or etching have been employed as dry etching techniques to accomplish, among other things, etching of semiconductor layers. In general, processing is carried out by establishing a DC or rf generated plasma with an inert atmosphere, such as Ar. An example of such processes are disclosed in U.S. Pat. No. 4,622,094. These techniques generally involve the formation of a plasma and the physical removal of materials from the semiconductor surface due to bombardment of the surface with ions. However, the accuracy of these techniques has left much to be desired, particularly relative, for example, to improvements to pattern resolution, surface morphology, attained anisotropy, etching depth and reduced mask erosion.
On the other hand, in the case of such dry etching, for example, ion etching employing an inert gas medium, such as Ar, in order to enhance the etching rate to a level of practical utilization, it is necessary to increase the plasma discharge power. However, this, in turn, results in substantial damage to a semiconductor materials.
More recently, there has been an increased interest in reactive dry etching techniques because these techniques, as compared to the above mentioned previous techniques, promise better pattern resolution in submicron large scale integration providing a higher degree of circuit density with improvements in surface morphology, increased anisotropy, lower thermal stress due to lower temperature processing, higher plasma densities at lower pressures, enhanced etching rates, enhanced selectivity ratio, deeper etching capability and reduced mask erosion. Further, they eliminate the need for the above mentioned post etching treatment steps employed in wet etching and improved to a great degree the accuracy can be achieved in the resultant linewidth and pattern resolution. As a result, semiconductor structure yields may be made more uniform and higher. These techniques generally involve the chemical removal of materials from the semiconductor surface or a combination of chemical and physical removal from the semiconductor surface comprising atoms or molecules of etched materials and products of the reaction between surface molecules and the reactive gas species.
Dry etching techniques include reactive ion etching (RIE), ion beam assisted etching (IBAE) and hot jet etching (HJE), and reactive ion beam etching (RIBE), such as microwave plasma dry etching, each of which involves a chemically reactive vapor or gaseous species, for example, comprising a halogen, such as F.sub.2, Br.sub.2 or Cl.sub.2, in a vapor phase compound. In RIE, the sample or target to be etched is placed on a cathode in an electric field established between an anode and cathode in the presence of a selected flux of a chemically reactive species that reacts with atoms or molecules on the surface of the sample. The potential applied between the anode and cathode is sufficient to ionize atoms or molecules in the gas as well as produce radicals. The positively charged ions produced in the plasma are attracted to the cathode and upon impact physically remove or etch away material from the sample surface. The reactive species will also chemically react with atoms or molecules on the surface of the sample which are also removed by the incident ions on the surface of the sample. As an example of RIE, see U.S. Pat. No. 4,640,737.
In the case of reactive ion etching (RIE) employing a reactive gas, such as BCl.sub.3, damage to the semiconductor materials is, to a degree, less compared to that of ion etching. In any case, damage to the surface of semiconductor materials under this etching treatment is still major and not acceptable. In order to reduce the damage, the gas pressure of the etching system may be raised while lowering the discharge power. However, the ion sheath width and the mean free path of the ions and neutral particles become almost the same, causing the beam of ions to lose directionality thereby increasing the potentiality of isotropic etching to occur. Thus, RIE provides a significant drawback to dry etch processing particularly for Group II-VI compound semiconductors.
In IBAE, a combination of ions from an inert gas, e.g., Ar.sup.+, from an ion beam source and a flux of chemically active species, e.g., F or Cl, are directed to the sample and by control of the ion beam and the reactive species, a controlled anisotropic etching can be carried out. In the case of HJE, there is no ion beam employed and a flux of reactive radicals is formed and directed onto the sample. See, for example, the articles of M. W. Geis et al.: "A Novel Anisotropic Dry Etching Technique", Journal of Vacuum Science Technology, Vol. 19(4), pp. 1390-1393, November/December, 1981; "Hot-Jet Etching of Pb, GaAs, and Si", Journal of Vacuum Science Technology, Vol. B5(1), pp. 363-365, January/February, 1987; and "Summary Abstract: Etching With Directed Beams of Ions or Radicals", Journal of Vacuum Science Technology, Vol. A5(4), pp. 1928-1929, July/August, 1987. Also, see U.S. Pat. No. 4,874,459 relative to a modified IBAE method as well as a summary of other reactive dry method techniques mentioned in the background of this reference.
In RIBE, the source of ions (e.g., Cl.sup.+) and radicals (e.g., Cl*) is generally formed in and extracted out of a separate chamber and accelerated via an ion extraction grid or electrode into the etching chamber. See, for example, the article of K. Asakawa et al., "GaAs and GaAlAs Equi-Rate Etching Using a New Reactive Ion Beam Etching System", Japanese Journal of Applied Physics, Vol. 22(10), pp. L653-L655, October, 1983. Electron Cyclotron Resonance (ECR) microwave plasma source is employed which provides for higher efficiency in plasma generation and higher generation of reactive species achieving improved anisotropy and higher etching rates. Other examples are found in U.S. Pat. Nos. 4,795,529; 4,778,561; 4,609,428; 4,859,908 and 4,734,157.
Thus, reactive dry etching processes generically provide a source of reactive species in the form of either reactive ions, e.g. Cl.sup.+, or reactive radicals, e.g. Cl*, or a combination of reactive ions and radicals forming a reactive flux, e.g. Cl.sup.+ and Cl*, or a source of reactive species assisted by other ions, e.g. Cl.sup.+ and/or Cl* in combination with Ar.sup.+, that are generated, focussed and/or accelerated to the sample target to provide a chemical action at the sample surface with surface molecules and sputter or otherwise remove reaction products from the sample surface via the outlet affluent.
It is of importance to note that all of the foregoing references relating to various dry etching techniques are methods that have specifically evolved for the purpose of etching Group III-V materials, e.g., GaAs and AlGaAs. The techniques have not been generally applied to Group II-VI compound semiconductors, such as ZnSe, ZnS, or ZnS.sub.x Se.sub.1-x, because the developed treatments, as reported in these references, have not been designed for these compounds and their attempted application according to their specific teachings would provide etching rate that are not of a practical level and would result in damage to the crystalline structure without good anisotropy. Further, problems persist in the utilization of these etching techniques of the prior art, particularly in the case of Group II-VI compound semiconductors wherein both selective wet etching and dry etching techniques have been used employing a mask comprising an insulation film, such as photoresist, SiO.sub.2, or the like. In particular, good anisotropy has not been obtained, particularly in connection with masking techniques, and good selectivity ratio has not been achieved.
It is an object of this invention to provide an improved method of reactive ion beam etching (RIBE).
It is another object of this invention to provide a reactive ion beam etching (RIBE) method particularly suitable for Group II-VI compound semiconductors.
It is another object of this invention to provide modified reactive ion etch methods incorporating precleaning treatment to the semiconductor structures to be treated providing high anisotropy, enhanced etching properties and much improved surface morphology, particularly for Group II-VI compound semiconductors.
It is a further object of this invention to manufacture semiconductor structures employing Group II-VI compound semiconductors with excellent reproducibility by providing etching methods of Group II-VI compound semiconductors with high reproducibility, excellent practical use with no or negligible damage to etched semiconductor materials and with a capability of producing anisotropically patterns previously not realizable in the prior art.